The present invention relates to a method for operating a digital mobile radio network using space-time block codes (ST codes).
Such methods are for example disclosed in “Space-Time Codes for High Data Rate Wireless Communication Performance Criterion and Code Construction” by V. Tarokh et al. in IEEE Transactions on Information Theory, vol. 44, No. 2 Mar. 1998, pages 744-765 or in “Space-Time Block Codes from Orthogonal Designs”, IEEE Transactions on Information Theory, vol. 45, No. 5, July 1999, pages 1456-1467 by V. Tarokh et al.
Use of the time-space block codes disclosed there is subject to the following physical problem in mobile radio networks: the high level of attenuation or distortion caused by the transmission path of a transmission signal transmitted in a wireless manner from a transmitting station (e.g. a fixed base station in a mobile radio network) makes it very difficult for a receiving station (e.g. a mobile station in a mobile radio network) to identify the originally transmitted transmission signal correctly. This applies in particular, when the receiving mobile station is located in a multipath wireless environment, where it only receives a plurality of highly attenuated echoes of the original transmission signal due to multiple reflections of the original transmission signal off the walls of surrounding buildings. To rectify this, a certain “diversity” of the received signal has to be ensured. This is achieved by providing the receiving station with one or more additional, less highly attenuated “images” of the transmitted transmission signal. What is known as the “diversity order” here is a measure of the number of statistically independent “images” of the transmission signals transmitted by the transmitter received at the receiver.
In practice, this necessary diversity is achieved, for example, by using a plurality of transmitter or receiver antennas, each positioned spatially apart from each other or with different polarities on the transmitter and/or receiver side, each of which transmits or receives an “image” of a signal to be transmitted. “Image” here does not necessarily mean that two or more exactly identical copies of the same signal are transmitted.
Instead, even with the known space-time block codes, different signals can be transmitted at a defined time from the different transmitter antennas, each of the signals being created by a specific algorithm from the data bits of the original signal. Each receiver antenna then receives the sum of the transmission signals, modified due to the transmission path, at a defined (later due to the signal runtime) time, from which signals those data bits, which correspond with the greatest likelihood to the data bits of the original signal, are reconstructed on the receiver side using MLD (maximum likelihood detection) estimate algorithms.
See FIGS. 1 and 2 for a clearer explanation of the problem.
FIG. 1 shows a diagrammatic illustration of a known single antenna to single antenna radio transmission link between a transmitter Tx and a receiver Rx in a digital mobile radio network.
Data bits are fed into the transmitter, comprising, for example, a series of ones and zeros. Let it be assumed that a bit vector (1,0,1) of length l=3 is fed in, comprising the series 1-0-1. Depending on the coding method used, this bit vector is transformed on the transmitter side with one-to-one correspondence (i.e. with reversible uniqueness) into a symbol vector of length n. As shown in FIG. 1 by different shades of gray, the figures occurring in the symbol vector do not have to be 1 and 0. In particular, the figures occurring in the symbol vector can also be complex figures, with which for example the signals corresponding to the real component and the imaginary component are sent with a phase offset of 90° in respect of each other.
FIG. 1 shows the case where n=l=3, i.e., each individual bit is coded into an individual symbol (BPSK modulation—binary phase shift keying; with QPSK—quadrature phase shift keying n=l/2 would apply).
A symbol vector comprising a plurality of individual symbols is then transmitted by an antenna in the transmitter Tx and received by a receiver Rx. The symbol vector of length n=3 is reconstructed there by a reverse transformation into the original bit vector (here (1,0,1)) of length l=3.
FIG. 2 shows a diagrammatic illustration of a radio transmission link between a transmitter unit with n=3 transmitter antennas Tx1, Tx2, Tx3 and a receiver Rx in a digital mobile radio network, with which space-time block codes already known per se are used. A bit vector of length l=3 is fed to a space-time block coding device (ST coder=space time coder). This maps an incoming bit vector onto an n×n matrix.
Here, n corresponds to the number of transmitter antennas used in the transmitter unit. In a defined time slot j an antenna i sends a signal, which corresponds to the matrix element cijk of a 3×3 matrix Ck, which was coded by the ST coder from the incoming vector of length l=3. k here is an index, which differentiates individual matrices, which in turn correspond for their part to k different bit vectors. These relationships are described in more detail below.
As a result of the signals transmitted via the three transmitter antennas Tx1, Tx2 and Tx3, signals arrive at the receiver antenna Rx at a receiving time corresponding to the transmission time slot j (later due to runtime), where the signals correspond to the matrix elements c1jk, c2jk, c3jk. On the receiver side, a space-time block matrix Ck is reconstructed from the sum of the incoming signals in a space-time block decoding device (ST decoder) by MLD algorithms and translated back by reverse mapping into the corresponding original bit vector (here (1,0,1)).
The general problem here is maximizing diversity at the mobile station of a digital mobile radio network by using a plurality of transmitter antennas at the base station. No specific prior knowledge about the downlink channel (from a base station to a mobile station), which changes over time, should be assumed.
In the case of linear space-time block codes with two antennas, the result is known (it is optimal in the sense that it doubles the diversity for two antennas) and it has become part of the UMTS standard (3GPP TS 25.211 V3.4.0: Physical channels and mapping of transport channels onto physical channels (FDD) (1999 edition), September 2000). This known space-time block code scheme satisfies the “rate 1” requirement.
A “rate 1” space-time block code scheme can be described as a system, in which, for each time interval considered, precisely the same number of data bits can be sent through effectively from a transmitter to a receiver as with the reference system shown in FIG. 1 with only one transmitter and one receiver antenna. In other words, a “rate 1” ST code has the same transmission rate compared with a basic system with only one transmitter and one receiver antenna. This is for example a system in which a block of two code words is transmitted to the receiver at the same time in two successive time windows via two different transmission channels; the receiver thereby receives precisely the same amount of information per unit of time as if two corresponding individual bits had been transmitted in two successive time windows via a single transmission channel. Provision is therefore made in the W-CDMA mode of the UMTS standard for future mobile radio systems (see for example www.3GPP.org) to transmit the standard mapping of four bits onto 24=16 ST symbols in two time stages via two transmitter antennas. A “rate 1” system for external blocks therefore behaves in exactly the same way as the basic system with only one transmitter and one receiver antenna. This characteristic is decisive when upgrading a basic system to a system with a plurality of transmitter antennas and space-time block codes.
The space-time block code used in W-CDMA mode corresponds to what is known as the Alamouti code. This is a code that is very simple to reconstruct on the receiver side to increase diversity in a digital mobile radio network with a transmitting station with two (n=2) transmitter antennas. The Alamouti code is for example disclosed in “A simple Transmitter Diversity Scheme for Wireless Communications”, IEEE J. Select Areas Commun, vol. 16, pages 1451-1458, October 1998 by S. M. Alamouti or in the two publications by V. Tarokh referred to above.
In order to increase diversity in a mobile radio network further in the future both in the uplink direction (from a mobile station to a base station) and in the downlink direction (from a base station to a mobile station), the number of antennas per sector of a base station should be greater than two. It is therefore clear that space-time codes are required, which can be used in the case of three, four or more transmitter antennas. An increase in diversity with the same transmission power results in an increase in receiving quality. Or, looked at another way, an increase in diversity with the same receiving quality means a reduction in transmission power. The transmission power then not used up in a transmitter can in turn be used to supply more users.
The performance of a space-time block code is also influenced by the intervals between the code words. Observations on this are contained in the two articles by V. Tarokh referred to above.
In the transmission shown diagrammatically in FIG. 2 the matrix shown with the elements cijk corresponds to a code word with the number k. The “interval” between code words, i.e. the “interval” between two matrices Ck1 and Ck2 is ultimately a measure of the quality of a transmission code, i.e., the likelihood with which the originally transmitted code words can be reconstructed as uniquely as possible from a sequence of code words received at the receiver with distortion due to the transmission path. In this respect the known space-time blocks in the case of two transmitter antennas can be further improved.
In the case of three, four or more antennas (n>2) it is shown in “Space-time Block Codes from Orthogonal Designs”, IEEE Transactions on Information Theory, vol. 45, No. 5, July 1999, pages 1456-1467 by V. Tarokh et al. that linear codes with “rate 1” and complex symbols cannot exist.
In a presentation given at Globecom 2000 in December 2000 on “Complex Space-Time Block Codes for Four Tx Antennas” by Olav Tirkkonen and Ari Hottinen, the case of n=4 transmitter antennas was examined and complex space-time block codes specified with a rate of ¾.
The search for space-time codes, in particular for a number n>2 of transmitter antennas, has therefore taken two directions:
1. In “Space-time Block Codes from Orthogonal Designs”, IEEE Transactions on Information Theory, vol. 45, No. 5, July 1999, pages 1456-1467 by V. Tarokh et al., linear space-time codes with a rate less than 1 are constructed for more than two transmitter antennas. Here the space-time symbols are linear combinations of the original signals. These constructions are not available for external coding, because they do not have the “rate 1” characteristic. This means that they cannot be integrated as simple additional features in an existing mobile radio system without space-time coding.
2. Space-time trellis codes are constructed by combining space-time codes with external error correction codes. Combining space-time mapping with external coding however makes it impossible to integrate space-time codes as a simple additional feature in an existing system. Also in such a case proven external coding techniques such as turbocodes (see C. Berron, A. Glavieux and P. Thitimajshima: Near Shannon limit error correcting codes and decoding: turbo codes” in Proc. IEEE ICC, Geneva, May 1993, pages 1064-1070)) or trellis codes must be modified.
This reveals a problem in that the digital code words to be transmitted between the transmitting and receiving stations in a digital mobile radio network should be optimized in the case of two or more transmitter antennas with regard to the following:
1. There should be a “rate 1” code if possible.
This is a mandatory requirement in order to be able to upgrade mobile radio networks already commercially available with the lowest possible upgrade costs for the use of the new space-time block codes. When “rate 1” is used when upgrading from a known single antenna to single antenna system shown in FIG. 1 to a multi-antenna to single antenna system as shown in FIG. 2 (or even to multi-antenna to multi-antenna systems), the assemblies (not shown in FIGS. 1 and 2) required on the transmitter side to generate the bit vector to be fed in and the assemblies (also not shown) required on the receiver side to further process the reconstructed bit vectors output by the receiver unit do not have to be changed. “Rate 1” codes thus guarantee “downward compatibility” of digital mobile radio stations operated with multi-antenna units and corresponding space-time block codes with other already existing system components, thereby eliminating a significant obstacle to investment impeding the practical introduction of “rate≠1” systems, as with these the assemblies for transmitter-side bit vector generation and for receiver-side bit vector reconstruction must also be changed.
The introduction of “rate 1” space-time blocks means that it is also possible to leave the other parameters of a transmitter code scheme (such as channel coding, interleaving, service multiplexing, etc.) unchanged.
2. The code should be simple to construct on the transmitter side and simple to reconstruct on the receiver side.
3. The code words should have the “biggest interval” possible between them. This means that a set of code words should be structured so that from the signals received on the receiver side with noise and/or distortion, each of which comprises the original signal transmitted on the transmitter side times a “fading factor” describing the fading of intensity associated with increasing distance plus noise (thermal noise at the input amplifier of the receiver plus interference noise due to disruptive signals from other users of the mobile radio network), the original signal can be reconstructed even with a relatively high level of interference in the most error-free possible manner as the signals transmitted on the transmitter side (i.e. without confusion between individual code words).
4. The space-time block code used should maximize diversity, i.e., for two transmitter antennas the theoretically maximum degree of diversity 2 should be achieved if possible, for three transmitter antennas the theoretically maximum degree of diversity 3, etc.
5. The space-time block code used should allow complex transmission symbols, in order for example that it can be used with UMTS where QPSK (quadrature phase shift keying) modulation is used. Complex symbols also allow 8-PSK (8-phase shift keying) or M-QAM (M-fold quadrature amplitude modulation).